C.H.Ling wrote:
> ...
>
> Just found another phenomenon, when I shot a dark frame at the same ISO and
> shutter speed (ISO125, 1/8s) of the problem image. The dark frame came out
> totally dark even I push it two stops so I can't use the dark frame
> subtraction method to cancel the pattern.
>
> The pattern noise only appear on the area that is not totally dark so it
> doesn't like a hardware (circuit noise) problem, may be a firmware update can
> really solve it?
>
As I think about this, I wonder if the effect is the same regardless of
the resolution at which the shot is taken. A repeating pixel level
pattern could result from the way adjacent pixels are merged (I assume
they are merged?) for the smaller sizes, like the 5.2 MP version you
posted. If so, a firmware solution may be possible.
I didn't expect dark frame subtraction to help. That's a different sort
of issue without any pattern.
I think it was AG, earlier in this thread who suggested it could be a
demosaicing artifact. I could see how that could be the case, although I
don't see any meaningful difference using the three algorithms available
in RawTherapee.
Down at the bottom of the brightness range, the nature of linear digital
coding means that there are very few different numerical values
available to represent shades of brightness.
Whatever the analog ability of the sensor to react to tiny differences
in light, the analog voltages have to be encoded digitally. Perhaps this
is some sort of quantization effect where tiny diffrences in voltage are
forced into "buckets" with average values that create too much contrast
between pixels. This could magnify any subtle pattern of sensor site
sensitivity differences resulting from physical sensor design.
If that's the source, it may require a hardware changes, perhaps to the
Ato D converters.
(Now that I think of it, at what point do the sensor sites start showing
the effects of the number of individual light quanta that they sense?)
Moose
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